1. Midf players also help in defense so it's good to have some def. skills too
My logic in those players is to have in attack some more skills (+5-10 % above players avg quality), def. less (-5-10% less skills) and M&P equal to q avg.
2. of course it's useful - for every M and AM player
3. In 4-5-1V the AMs scoring more than the ML-MR. It helps cause usually ST doesn't score much.
If oppo also has AMs or has a very strong team or you want to keep a good score of a 1st round (in KO games), then 4-1-4-1 is better to keep up your defense.
4. If the player is not a young fast trainer and you don't have 30 vids/day, then it's not worthy to add a new position. Better add to quality.
I don't use DML-DMR positions so don't know much about it.
I like AML-ML & AMR-MR so to switch formations or coach better my games.
If I can't find an AML-ML but only ML young fast trainer, then I give also AML position.